This invention relates to the transmission of electronic signals, and more particularly, to the transmission of electronic signals between transmission lines having different characteristic impedances.
In modern electronic systems, signals are often transmitted on transmission lines having different characteristic impedances. For example, when a processor mounted on a printed circuit board sends an electronic signal to a memory module mounted on the printed circuit board, the signal may travel on the circuit board over a transmission line having a characteristic impedance of 50 ohms, and on the memory module over a transmission line having a characteristic impedance of 28 ohms.
When transmission lines having different characteristic impedances are connected together in an electronic system there is an impedance mismatch at the point where the transmission lines are connected. When an electronic signal traveling along a transmission line encounters an impedance mismatch, the electronic signal may be reflected back along the transmission line. At low data rates, and as long as reflections die out before another signal is transmitted, reflected signals generally do not adversely affect the operation of the system. However, at high data rates, reflected signals can cause an electronic system to operate unpredictably or to fail.
Several methods are available for reducing reflections on coupled transmission lines in electronic systems. The methods are generally divided into active methods and passive methods. In the active methods, an active device, such as a transistor, is inserted between two transmission lines that have different characteristic impedances. The impedance of the input port of the transistor is designed to be about equal to the characteristic impedance of the transmission line to which the transistor is attached. Signals that arrive at the input port of the transistor are not reflected because the impedance of input port of the transistor matches the characteristic impedance of the transmission line. Active devices can be designed to match most transmission line impedances. In the passive methods, a passive device, such as a transformer, is inserted between two transmission lines that have different characteristic impedances. The impedance of the input port of the transformer is designed to be about equal to the characteristic impedance of the transmission line to which the input port of the transformed is attached. Passive devices can be designed to match many transmission line impedances.
Unfortunately, both passive and active impedance matching methods have disadvantages. Active devices consume power and take up a large amount of surface area on a printed circuit board or other substrate. Passive devices also take up a larger amount of surface area on a printed circuit board or other substrate. Both passive and active devices are expensive because they require mounting additional components on a printed circuit board or other substrate. In addition, both active and passive methods are relatively unreliable because they usually require soldering to couple the devices to a circuit board and to couple to the devices to the transmission lines. Solder joints usually have a high failure rate when compared to the failure rates of electronic components.
For these and other reasons there is a need for the present invention.